On Mon, Aug 07, 2017 at 02:17:33PM -0400, Prarit Bhargava wrote:
>
>
> On 08/07/2017 01:14 PM, Luis R. Rodriguez wrote:
>
> >
> > Note printk_late_init() is a late_initcall(). This means if the
> > printk_time_setting was disabled it will take a while to enable it. Enabling it
> > is done at the device_initcall(), so if printk setting is disabled but a user
> > enables it with a toggle of the module param there is a period of time during
> > which time resolution would be different.
>
> I'm not sure I follow your comment. Could you elaborate with an example of
> what you think is going wrong or might be confusing?
Sure let's consider this:
+static u64 printk_get_ts(void)
+{
+ u64 mono, offset_real;
+
+ if (printk_time <= PRINTK_TIME_LOCAL)
+ return local_clock();
+
+ if (printk_time == PRINTK_TIME_BOOT)
+ return ktime_get_boot_log_ts();
+
+ mono = ktime_get_real_log_ts(&offset_real);
+
+ if (printk_time == PRINTK_TIME_MONO)
+ return mono;
+
+ return mono + offset_real;
+}
So even if printk_time was flipped in the end the backend routines used will be
local_clock(), ,ktime_get_boot_log_ts() or ktime_get_real_log_ts().
This is used here;
@@ -1643,7 +1756,7 @@ static bool cont_add(int facility, int level, enum log_flags flags, const char *
cont.facility = facility;
cont.level = level;
cont.owner = current;
- cont.ts_nsec = local_clock();
+ cont.ts_nsec = printk_get_ts();
cont.flags = flags;
}
But lets inspect these new calls:
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
@@ -477,6 +479,24 @@ u64 notrace ktime_get_boot_fast_ns(void)
}
EXPORT_SYMBOL_GPL(ktime_get_boot_fast_ns);
+u64 ktime_get_real_log_ts(u64 *offset_real)
+{
+ *offset_real = ktime_to_ns(tk_core.timekeeper.offs_real);
+
+ if (timekeeping_active)
+ return ktime_get_mono_fast_ns();
+ else
+ return local_clock();
+}
+
+u64 ktime_get_boot_log_ts(void)
+{
+ if (timekeeping_active)
+ return ktime_get_boot_fast_ns();
+ else
+ return local_clock();
+}
+
So they are really only effectively calling something other than
what lock_clock() returns *iff* timekeeping_active is true. But
this is only set later at the respective device_initcall() in this
file:
@@ -1530,6 +1550,8 @@ void __init timekeeping_init(void)
write_seqcount_end(&tk_core.seq);
raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
+
+ timekeeping_active = 1;
}
So when the boot param is processed and prints out that it has
changed someone inspecting any time setting after that print
may assume its using after that ktime_get_mono_fast_ns() or
time_get_boot_fast_ns() but this is not accurate, it will use
local_clock() until *after* device_initcall().
So in between boot and this particular device_initcall() time
resolution can only be local_time(). Seems worth documenting
that.
Luis
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